The present invention relates to an optical information recording medium used for recording information in the form of a series of pits formed by a laser beam, and more particularly, to an improved material for use as an optical-information recording layer.
The optical information recording medium, such as an optical disk, has an advantage of storing a large capacity of information owing to its high storage density. The recording medium includes at least a substrate and a recording layer of a low-melting point material disposed on the substrate. As the material of the recording layer, tellurium (Te) or its compound was proposed in U.S. Pat. Nos. 3,971,874 (Ohta et al), 4,373,004 (Asano et al), 4,385,376 (Takaoka et al) and 4,433,340 (Mashita et al). Another example of materials for the recording layer was tellurium-selenium (Te-Se) alloy which is disclosed in U.S. Pat. Nos. 4,214,249 (Kasai et al), 4,238,803 (Terao et al), 4,348,461 (Terao et al), 4,385,305 (Terao et al) and 4,434,429 (Terao et al).
From the viewpoint that pits formed in the recording layer have a good shape, Te or Te-Se alloy is preferable owing to its low surface tension. However, a large power is necessary for the Te or Te-Se alloy recording layer to open a pit therein and therefore Te or Te-S has not been fully acceptable for achieving high degrees of recording sensitivity and a recording power margin.
Further, it is required for the recording medium for use in an optical information recording and reading system to prevent the excessive expansion of the size of the pit formed in the recording layer during a recording operation, in order that the focusing and tracking servo systems included in the recording and reading system operate stably. However, the pit size becomes inevitably large in the recording layer since high laser power is required to form the pit in the Te or Te-Se recording layer. This results in a difficulty in providing high stability in operation of the servo systems.